feat: file-size threshold and observation dedup for timeline gate

- Skip gate for files under 1,500 bytes — timeline (~370 tokens) costs
  more than just reading small files directly
- Deduplicate observations by memory_session_id (one per session)
- Rank by specificity: files_modified > files_read, fewer tagged files > many
- Fetch 40 candidates, dedup/score down to 15 for display
- Reduce default by-file query limit from 30 to 15

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/services/sqlite/observations/get.ts

@@ -122,7 +122,7 @@ export function getObservationsByFilePath(
   options?: { projects?: string[]; limit?: number }
 ): ObservationRecord[] {
   const likePattern = `%${filePath}%`;
-  const limit = options?.limit ?? 30;
+  const limit = options?.limit ?? 15;
   const params: any[] = [likePattern, likePattern];
 
   let projectClause = '';

src/services/worker/http/routes/DataRoutes.ts

@@ -116,7 +116,7 @@ export class DataRoutes extends BaseRouteHandler {
 
   /**
    * Get observations associated with a file path, scoped to projects
-   * GET /api/observations/by-file?path=<file_path>&projects=<comma,separated>&limit=30
+   * GET /api/observations/by-file?path=<file_path>&projects=<comma,separated>&limit=15
    */
   private handleGetObservationsByFile = this.wrapHandler((req: Request, res: Response): void => {
     const filePath = req.query.path as string | undefined;